This invention relates to a compressor control system. More specifically, this invention relates to a compressor surge control system for estimating, correcting and eliminating surge.
Compressor surge control systems, also known as anti-surge controllers, use a standard PID controller for regulating a recycle valve of the compressor when flow rate decreases below a predefined set point. The minimum set point for recycling is established based on rules of thumb and operating guidelines and is typically set at a fixed margin from the surge limit line. (or minimum flow set point).
A compressor surge line is defined by the compressor manufacturer consisting of several points for various operating conditions. The compressor surge line is typically a curve that is configured based on either field testing or calculated using the given performance maps. A mathematical function curve or a two dimensional lookup table is used to store the points defining the surge line of a compressor in computer memory. However, the compressor surge line can change due to variations in gas composition, suction temperature, speed, inlet geometry, and the like causing problems in the art.
As a result of this problem a surge point or surge line of a compressor is defined to account for variations in gas compositions, suction temperature, speed, inlet geometry, and the like. Currently in the art compressor surge controllers employ a surge parameter based on polytropic head and volumetric flow. Compressor surge is detected based on rate of change of compressor flow or discharge pressure to exceed a defined threshold or compressor operating point crossing the defined surge point or curve in the surge controller.
Still, problems remain with these types of controllers. Specifically, if the compressor surge point is not tested by the compressor vendor during shop testing then the surge line provided by the compressor manufacturer is typically an estimate of the actual surge point. Using an estimated surge point and not validating the curve in the field typically results in either the actual compressor surge point being to right or the left of the estimated curve provided by the vendor. In addition, the compressor surge line can shift due to performance degradation from impeller fouling, internal recycling, and inter-stage cooler plugging or due to significant changes in gas molecular weight or inlet temperature from the compressor design data. Therefore, any shifts in compressor performance can potentially lead to compressor surging and the surge control system must be able to detect the onset of surging and eliminate repetitive cycles of surging to occur.
Sometimes a compressor can also surge due to a stuck surge valve or incorrect control tuning parameters configured by the field engineers. Several continuous surge cycle events can lead to damage of the compressor due to bearing failures, temperature buildup, excessive vibration, impeller tip rubbing the housing, and over-speed. Existing surge control systems provide a trial and error method to correct for inaccurate surge line configuration or shifts in surge point. These methods are based on arbitrary increases in the surge control margin for each occurrence of surge cycle detection to alleviate surge condition. As a result, if the required correction to surge margin is set incorrectly then multiple cycles of surging can result and potentially damage the compressor. It is also possible that required correction to surge margin is excessive, thereby causing excessive recycling and process upsets. Moreover, there are no defined guidelines available to a field engineer to configure the required correction margin if a compressor surges to the right of the surge line defined in the controller.
Therefore, a principal object of the present invention is to provide an improved control system for a dynamic compressor that accounts for actual operating conditions of a compressor.
Yet another object of the present invention is to provide an improved control system that minimizes surge within a compressor.
These and other objects, features, or advantages will become apparent from the specification and claims.